1. Field of the Invention
The present invention relates to a method for preparing a layered structure including an oxide superconductor thin film, and more specifically to an improved method for preparing a layered structure including an oxide superconductor thin film and a dielectric thin film and/or an insulator thin film, which has a clear interface with negligible interface states, high crystallinity and excellent properties. The method is preferably applied to forming a gate structure of a superconducting field effect transistor including a superconducting channel of oxide superconductor, a gate insulator and a gate electrode stacked in the named order.
2. Description of Related Art
Various processes, such as reactive co-evaporation which is one of molecular beam epitaxy (MBE), pulsed laser deposition (PLD), sputtering, etc. are studied to prepare oxide superconductor thin films. Each process has features and drawbacks, and oxide superconductor thin films formed by each process have characteristics. Therefore, the selection of the process is determined by a use of the oxide superconductor thin film.
For example, a high quality oxide superconductor thin film having excellent superconducting properties, high crystallinity and a smooth surface can be prepared by the reactive co-evaporation. However, it takes a long time to prepare an oxide superconductor thin film by the reactive co-evaporation. The pulsed laser deposition has a fast deposition rate. However, an oxide superconductor thin film prepared by the pulsed laser deposition has a rather rough surface and rather low surface crystallinity. The sputtering has characteristics intermediate between the reactive co-evaporation and pulsed laser deposition. It has a deposition rate intermediate between those of the reactive co-evaporation and pulsed laser deposition. An oxide superconductor thin film prepared by the sputtering has a quality intermediate between those of the reactive co-evaporation and pulsed laser deposition.
The oxide superconductor thin film is used for superconducting devices utilizing the oxide superconductor material. One of the most important three-terminal superconducting devices is a field effect transistor type superconducting device (abbreviated as super-FET hereinafter) having a channel of a superconductor formed between a source and a drain. In this superconducting device, a current flowing through the superconducting channel is controlled by a signal voltage applied to a gate formed above the superconducting channel.
The super-FET mentioned above is a voltage controlled device which is capable of isolating output signals from input ones and of having a well defined gain. In addition, it has a large current capability.
A layered structure including an oxide superconductor thin film and another thin film is utilized for the above super-FET and the tunnel type Josephson junction device. In the super-FET, a depletion region generated in the oxide superconductor thin film by means of an electric field penetrating from a surface of the oxide superconductor thin film is utilized. In the tunnel type Josephson junction device, tunnel current flowing through the layered structure is utilized. The oxide superconductor thin films of the layered structures of these devices are required to have uniform superconductivity through the entire thickness.
In order to prepare a layered structure by stacking a thin film of an oxide material on an oxide superconductor thin film, the surface of the oxide superconductor thin film should be smooth and clean at the atomic level and an atomic plane should be exposed in the surface. For this purpose, deposition processes utilizing molecular beams under ultra high vacuum backgrounds, such as MBE including reactive co-evaporation, etc. (abbreviated as molecular beam deposition processes hereinafter) are considered to be preferable. An oxide superconductor thin film having a crystal structure continuous to a surface, a smooth and clean surface can be prepared by the molecular beam deposition process.
A deposition rate of the above molecular beam deposition process is kept low to improve crystallinity of a deposited thin film. A partial pressure of an oxidation source is decreased as low as possible to prevent contamination of a growing surface of the deposited thin film and oxidation of vapor sources in the above molecular beam deposition process. These may cause inconvenience when a layered structure of an oxide superconductor thin film and an oxide insulator thin film stacked in the named order are prepared by continuously depositing these thin films by the molecular beam deposition process.
For example, if a reducing material, such as SrTiO3 (abbreviated as STO hereinafter), BaTiO3 (abbreviated as BTO hereinafter), BaxSr1xe2x88x92xTiO3 (0 less than x less than 1; abbreviated as BSTO hereinafter), is used for the insulator thin film of the above layered structure and the layered structure is formed by the molecular beam deposition process, oxygen of the lower layer of the oxide superconductor thin film is absorbed in the insulator during a long deposition process. In addition, oxygen is liable to escape from the oxide superconductor thin film since it is held at a high temperature for a long time. In case of the oxide superconductor thin film alone, oxygen is again introduced into the oxide superconductor thin film while a temperature is lowered. However, in case of the layered structure, the insulator thin film interrupts the re-introduction of oxygen. By all of these, oxygen deficits are generated in the oxide superconductor thin film, which decrease its critical temperature and ultimately, the oxide superconductor thin film loses its superconductivity.
Thus, the oxide superconductor thin film of the layered structure prepared by the molecular beam deposition process has poor superconducting properties at least at a portion which is in contact with the insulator thin film. Now this problem will be explained more definitely with reference to the accompanying drawings.